An optical fiber having a silica-based glass core containing an alkali metal is known as an optical fiber exhibiting low Rayleigh scattering loss and low attenuation. If an alkali metal is contained in the core region of an optical fiber preform, the viscosity of the core region can be lowered, whereby the network structure of silica glass will be unified when the optical fiber preform is drawn into a fiber, and consequently the Rayleigh scattering loss due to structural unevenness can be decreased.
WO 2004/020357 and US 2006/0130530A indicate a diffusion method as a way of doping an alkali metal into silica glass. In the diffusion method, vapor of materials, such as an alkali metal or alkali metal salt, is introduced into a glass tube, and the alkali metal is diffusion-doped onto the inner surface of the glass tube by heating the glass tube with an outside heat source or by generating plasma in the glass tube.
After doping the vicinity of inner surface of the glass tube with an alkali metal in the above-mentioned manner, the diameter of the glass tube is reduced by heating it. After such diameter reduction, the inner surface of the glass tube is etched by a certain thickness in order to remove transition metal elements such as Ni and Fe, which were simultaneously added when the alkali metal was doped. It is possible to keep the alkali metal elements to remain even if the transition metal elements are removed by etching the glass surface by a certain thickness, because the diffusion of the alkali metal elements is quicker than that of the transition metal elements. After such etching, the glass tube is collapsed by heating, whereby a core rod containing an alkali metal is made. The core rod containing an alkali metal may be provided with glass around the perimeter thereof so that a core region for making a core of an optical fiber may be formed as a whole.
An optical fiber preform is produced in a manner in which a cladding region having a refractive index lower than that of the core region containing an alkali metal is formed around the outside of the core region. An optical fiber can be produced by drawing the optical fiber preform by a well-known method.